Mountain View, Folk Music Capital Of The World
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mountain View, Folk Music Capital Of The World clear sky chart from 2005-11-06 to 2026-05-17.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	11% 7% 2 6% 4 4 4 4 5% 7% 46%	131563
	Night	11% 6% 2 6% 4 3 4 3 5% 7% 49%	131505
Transparency	Day	37% 5% 10% 25% 19% 4	135170
	Night	34% 4 10% 25% 21% 6%	135976
Seeing	Day	18% 13% 27% 24% 15% 4	17432
	Night	16% 12% 26% 28% 15% 2	27320

By Month

Percentage Cloud Cover SE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	13% 9% 3 7% 4 3 4 3 4 5% 45%	5289
	11th to 20th	Day	9% 8% 2 5% 3 3 3 2 4 4 57%	5310
	21st to end	Day	17% 7% 2 5% 3 3 2 2 4 5% 50%	5812
February	1st to 10th	Day	16% 10% 3 6% 3 3 3 3 4 5% 43%	1890
	11th to 20th	Day	14% 8% 3 6% 6% 4 4 3 4 5% 42%	1981
	21st to end	Day	14% 9% 3 7% 5% 4 5% 3 5% 6% 41%	1733
March	1st to 10th	Day	15% 12% 3 6% 4 3 4 4 5% 5% 40%	5046
	11th to 20th	Day	10% 5% 2 4 3 3 4 4 5% 6% 53%	5090
	21st to end	Day	11% 8% 3 7% 4 4 4 4 5% 7% 43%	5647
April	1st to 10th	Day	11% 7% 3 7% 5% 4 4 4 6% 6% 45%	2520
	11th to 20th	Day	13% 6% 2 7% 5% 5% 5% 4 6% 6% 42%	2494
	21st to end	Day	16% 9% 3 6% 5% 5% 4 3 5% 6% 38%	2520
May	1st to 10th	Day	13% 9% 3 7% 6% 5% 6% 4 6% 6% 35%	5525
	11th to 20th	Day	12% 7% 3 7% 5% 4 5% 4 6% 8% 38%	5926
	21st to end	Day	17% 8% 3 7% 5% 5% 4 4 5% 7% 35%	6771
June	1st to 10th	Day	8% 5% 2 6% 6% 5% 6% 5% 7% 8% 43%	2800
	11th to 20th	Day	4 3 2 6% 6% 6% 6% 5% 8% 10% 45%	2800
	21st to end	Day	4 4 1 4 4 5% 5% 5% 7% 11% 50%	2800
July	1st to 10th	Day	7% 5% 2 6% 5% 5% 6% 5% 7% 10% 41%	6000
	11th to 20th	Day	4 3 1 5% 4 4 5% 4 6% 10% 54%	5900
	21st to end	Day	6% 5% 3 7% 5% 5% 6% 5% 8% 10% 41%	6422
August	1st to 10th	Day	5% 3 1 4 3 3 4 3 5% 9% 60%	4234
	11th to 20th	Day	10% 6% 2 6% 4 4 5% 4 7% 9% 42%	3880
	21st to end	Day	5% 4 2 5% 4 4 4 4 7% 9% 52%	4312
September	1st to 10th	Day	10% 6% 2 5% 4 5% 5% 4 5% 7% 47%	2400
	11th to 20th	Day	13% 6% 2 4 5% 5% 4 3 6% 7% 45%	2400
	21st to end	Day	10% 5% 2 5% 4 3 4 3 5% 6% 53%	2106
October	1st to 10th	Day	7% 5% 2 5% 4 3 3 4 5% 6% 57%	2000
	11th to 20th	Day	13% 5% 1 4 3 2 3 3 3 6% 58%	2000
	21st to end	Day	19% 7% 2 4 2 3 3 3 4 5% 49%	2200
November	1st to 10th	Day	11% 6% 2 5% 4 3 2 2 3 5% 57%	2058
	11th to 20th	Day	9% 5% 2 6% 3 4 4 3 5% 7% 53%	1917
	21st to end	Day	15% 9% 2 6% 4 3 3 3 4 5% 46%	1820
December	1st to 10th	Day	11% 7% 3 7% 4 3 3 3 3 4 52%	2690
	11th to 20th	Day	11% 9% 2 7% 4 3 3 3 4 4 48%	3088
	21st to end	Day	14% 12% 3 8% 5% 4 5% 4 5% 5% 36%	4182
January	1st to 10th	Night	14% 9% 3 7% 3 3 3 2 4 6% 46%	7634
	11th to 20th	Night	9% 5% 2 6% 3 3 3 2 3 4 60%	7680
	21st to end	Night	18% 9% 3 7% 3 3 3 2 4 5% 43%	8402
February	1st to 10th	Night	15% 9% 3 6% 4 3 4 3 4 5% 42%	2730
	11th to 20th	Night	16% 7% 3 7% 4 3 3 2 5% 6% 44%	2522
	21st to end	Night	14% 6% 2 6% 4 3 4 3 5% 6% 47%	2076
March	1st to 10th	Night	17% 9% 3 7% 3 4 4 2 4 5% 43%	5043
	11th to 20th	Night	10% 8% 2 5% 3 3 3 3 5% 6% 52%	5090
	21st to end	Night	9% 7% 3 7% 5% 4 4 3 5% 7% 46%	5524
April	1st to 10th	Night	10% 7% 2 5% 4 4 4 3 5% 7% 49%	2100
	11th to 20th	Night	11% 7% 3 5% 3 3 4 3 5% 6% 48%	2075
	21st to end	Night	14% 6% 2 7% 4 4 3 4 6% 6% 44%	2100
May	1st to 10th	Night	16% 8% 3 6% 4 4 5% 4 6% 6% 39%	4590
	11th to 20th	Night	16% 8% 3 6% 4 4 4 4 5% 8% 37%	4118
	21st to end	Night	18% 6% 3 6% 5% 4 5% 3 5% 7% 38%	4088
June	1st to 10th	Night	7% 5% 1 4 4 3 4 3 6% 11% 51%	1600
	11th to 20th	Night	5% 3 1 4 4 3 4 3 6% 10% 58%	1600
	21st to end	Night	3 3 1 3 3 3 4 4 6% 12% 58%	1600
July	1st to 10th	Night	6% 4 2 6% 4 5% 5% 5% 8% 10% 46%	3972
	11th to 20th	Night	4 2 1 3 2 3 4 4 6% 10% 61%	4077
	21st to end	Night	4 5% 2 6% 5% 4 4 4 7% 10% 50%	4449
August	1st to 10th	Night	2 2 1 3 3 3 4 4 5% 10% 64%	2926
	11th to 20th	Night	4 3 1 5% 4 4 5% 4 7% 9% 53%	3166
	21st to end	Night	4 3 1 4 4 3 5% 4 6% 9% 58%	3586
September	1st to 10th	Night	8% 4 2 5% 4 3 3 3 6% 8% 55%	2000
	11th to 20th	Night	10% 6% 2 6% 3 3 5% 3 6% 7% 51%	2000
	21st to end	Night	8% 5% 2 5% 4 3 3 3 4 6% 59%	2270
October	1st to 10th	Night	6% 3 2 3 3 3 3 3 5% 7% 63%	2400
	11th to 20th	Night	11% 4 1 4 3 3 3 3 3 5% 61%	2400
	21st to end	Night	17% 6% 2 5% 2 3 2 3 4 4 53%	2640
November	1st to 10th	Night	10% 5% 2 4 3 3 3 2 4 6% 57%	2460
	11th to 20th	Night	10% 5% 1 4 3 3 4 3 4 6% 57%	2699
	21st to end	Night	13% 7% 2 4 3 4 4 3 5% 6% 50%	2797
December	1st to 10th	Night	11% 7% 2 5% 3 3 4 3 4 6% 53%	4720
	11th to 20th	Night	10% 8% 3 7% 4 4 4 3 4 5% 47%	5339
	21st to end	Night	17% 10% 3 7% 4 4 4 3 4 6% 38%	7032

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	43% 2 8% 28% 17% 2	5290
	11th to 20th	Day	33% 2 5% 22% 32% 6%	5310
	21st to end	Day	40% 2 6% 20% 25% 8%	5812
February	1st to 10th	Day	45% 4 9% 24% 15% 3	1890
	11th to 20th	Day	46% 4 9% 20% 18% 4	1991
	21st to end	Day	46% 3 8% 18% 18% 8%	1733
March	1st to 10th	Day	47% 4 6% 18% 13% 12%	5046
	11th to 20th	Day	32% 4 5% 23% 23% 13%	5090
	21st to end	Day	41% 4 9% 27% 18% 1	5649
April	1st to 10th	Day	39% 6% 9% 22% 21% 3	2520
	11th to 20th	Day	42% 5% 10% 21% 15% 7%	2494
	21st to end	Day	46% 5% 9% 22% 15% 2	2520
May	1st to 10th	Day	48% 6% 10% 19% 14% 2	5560
	11th to 20th	Day	42% 9% 12% 24% 12% 1	5930
	21st to end	Day	48% 7% 10% 18% 15% 2	6785
June	1st to 10th	Day	36% 9% 13% 24% 17% 1	2800
	11th to 20th	Day	30% 8% 16% 27% 15% 3	2800
	21st to end	Day	27% 8% 16% 28% 18% 4	2800
July	1st to 10th	Day	37% 5% 13% 28% 16% 1	5936
	11th to 20th	Day	25% 5% 15% 36% 17% 2	5831
	21st to end	Day	38% 3 16% 30% 11% 1	6240
August	1st to 10th	Day	21% 6% 16% 33% 20% 4	4220
	11th to 20th	Day	38% 5% 14% 30% 12% 1	3916
	21st to end	Day	28% 5% 12% 35% 16% 3	4311
September	1st to 10th	Day	35% 6% 11% 24% 18% 6%	2400
	11th to 20th	Day	37% 6% 11% 20% 23% 4	2400
	21st to end	Day	31% 6% 8% 23% 28% 4	2106
October	1st to 10th	Day	18% 3 5% 16% 41% 17%	3260
	11th to 20th	Day	20% 2 4 23% 38% 13%	3240
	21st to end	Day	33% 3 5% 32% 23% 5%	3538
November	1st to 10th	Day	33% 3 8% 25% 24% 7%	2058
	11th to 20th	Day	32% 4 7% 24% 23% 9%	1917
	21st to end	Day	41% 4 8% 20% 23% 4	1820
December	1st to 10th	Day	38% 2 7% 29% 22% 2	2690
	11th to 20th	Day	40% 3 6% 27% 21% 2	3085
	21st to end	Day	48% 4 8% 23% 13% 3	4182
January	1st to 10th	Night	42% 2 8% 30% 15% 3	7635
	11th to 20th	Night	29% 3 6% 19% 36% 7%	7680
	21st to end	Night	45% 2 7% 18% 22% 5%	8402
February	1st to 10th	Night	44% 4 9% 22% 17% 3	2730
	11th to 20th	Night	41% 5% 10% 23% 17% 5%	2523
	21st to end	Night	38% 4 10% 20% 20% 9%	2076
March	1st to 10th	Night	45% 5% 6% 15% 18% 12%	5043
	11th to 20th	Night	35% 4 6% 25% 19% 11%	5090
	21st to end	Night	38% 4 11% 28% 18% 2	5524
April	1st to 10th	Night	35% 5% 12% 23% 20% 5%	2100
	11th to 20th	Night	36% 5% 9% 23% 20% 8%	2075
	21st to end	Night	39% 6% 11% 24% 17% 3	2100
May	1st to 10th	Night	45% 7% 12% 24% 11% 2	4620
	11th to 20th	Night	44% 7% 13% 22% 11% 3	4119
	21st to end	Night	46% 6% 11% 21% 12% 4	4096
June	1st to 10th	Night	26% 7% 13% 30% 21% 2	1600
	11th to 20th	Night	22% 6% 13% 34% 21% 4	1600
	21st to end	Night	19% 7% 15% 32% 21% 7%	1600
July	1st to 10th	Night	32% 5% 17% 23% 22% 1	3929
	11th to 20th	Night	19% 6% 17% 39% 17% 2	4026
	21st to end	Night	30% 5% 17% 32% 15%	4320
August	1st to 10th	Night	17% 6% 17% 36% 22% 2	2916
	11th to 20th	Night	27% 4 21% 32% 15% 1	3196
	21st to end	Night	23% 5% 11% 35% 22% 3	3586
September	1st to 10th	Night	29% 5% 11% 29% 19% 8%	2000
	11th to 20th	Night	31% 6% 12% 21% 24% 5%	2000
	21st to end	Night	28% 4 9% 22% 31% 7%	2270
October	1st to 10th	Night	14% 3 6% 16% 38% 24%	3920
	11th to 20th	Night	18% 2 4 16% 37% 22%	3899
	21st to end	Night	28% 2 5% 28% 28% 9%	4252
November	1st to 10th	Night	30% 4 8% 24% 26% 8%	2460
	11th to 20th	Night	28% 3 7% 27% 25% 9%	2699
	21st to end	Night	37% 3 9% 23% 24% 4	2797
December	1st to 10th	Night	36% 2 7% 35% 17% 3	4720
	11th to 20th	Night	39% 3 7% 24% 21% 6%	5341
	21st to end	Night	47% 5% 9% 24% 12% 3	7032

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	32% 22% 31% 12% 3	330
	11th to 20th	Day	29% 28% 32% 10% 1	330
	21st to end	Day	30% 25% 35% 8% 3	363
February	1st to 10th	Day	30% 25% 32% 9% 3 1	330
	11th to 20th	Day	35% 26% 28% 8% 3 1	330
	21st to end	Day	37% 27% 25% 10% 1	273
March	1st to 10th	Day	31% 22% 33% 10% 3	330
	11th to 20th	Day	36% 19% 36% 9% 1	330
	21st to end	Day	27% 26% 36% 9% 2	389
April	1st to 10th	Day	26% 20% 31% 16% 6% 1	540
	11th to 20th	Day	23% 23% 33% 15% 6% 1	532
	21st to end	Day	25% 21% 31% 15% 7% 1	540
May	1st to 10th	Day	21% 20% 34% 19% 6% 1	543
	11th to 20th	Day	17% 15% 32% 21% 12% 2	741
	21st to end	Day	14% 10% 35% 26% 12% 4	789
June	1st to 10th	Day	11% 6% 24% 31% 22% 5%	730
	11th to 20th	Day	5% 4 25% 40% 20% 5%	730
	21st to end	Day	5% 4 28% 36% 22% 5%	730
July	1st to 10th	Day	8% 2 18% 34% 28% 10%	721
	11th to 20th	Day	5% 3 19% 36% 27% 9%	718
	21st to end	Day	7% 3 15% 35% 29% 12%	794
August	1st to 10th	Day	8% 2 17% 40% 25% 7%	726
	11th to 20th	Day	14% 2 13% 33% 29% 10%	538
	21st to end	Day	10% 2 14% 32% 30% 12%	583
September	1st to 10th	Day	11% 5% 14% 33% 31% 8%	530
	11th to 20th	Day	13% 3 13% 31% 27% 13%	530
	21st to end	Day	13% 9% 27% 27% 20% 5%	376
October	1st to 10th	Day	8% 9% 26% 38% 16% 2	330
	11th to 20th	Day	16% 16% 36% 22% 9% 1	330
	21st to end	Day	32% 13% 32% 18% 4 1	363
November	1st to 10th	Day	17% 16% 39% 19% 8% 1	330
	11th to 20th	Day	19% 18% 39% 17% 6% 1	330
	21st to end	Day	31% 18% 29% 17% 4 1	330
December	1st to 10th	Day	24% 28% 29% 13% 5% 1	330
	11th to 20th	Day	25% 24% 34% 13% 3	330
	21st to end	Day	27% 25% 37% 9% 1	363
January	1st to 10th	Night	22% 20% 32% 21% 4	1050
	11th to 20th	Night	22% 25% 32% 17% 3	1050
	21st to end	Night	22% 20% 35% 22% 2	1155
February	1st to 10th	Night	24% 20% 33% 21% 2	1050
	11th to 20th	Night	26% 21% 35% 17% 2	887
	21st to end	Night	25% 23% 31% 20% 2	692
March	1st to 10th	Night	24% 18% 29% 27% 3	644
	11th to 20th	Night	26% 14% 31% 25% 3	630
	21st to end	Night	22% 17% 34% 23% 5%	693
April	1st to 10th	Night	19% 19% 33% 24% 5%	630
	11th to 20th	Night	18% 20% 29% 25% 8%	623
	21st to end	Night	17% 15% 34% 25% 9%	630
May	1st to 10th	Night	14% 11% 32% 34% 8%	630
	11th to 20th	Night	17% 10% 28% 28% 16% 2	609
	21st to end	Night	10% 6% 28% 33% 20% 4	650
June	1st to 10th	Night	8% 6% 18% 34% 31% 4	600
	11th to 20th	Night	5% 2 19% 44% 27% 3	600
	21st to end	Night	5% 2 21% 38% 31% 5%	600
July	1st to 10th	Night	6% 2 11% 34% 38% 9%	585
	11th to 20th	Night	5% 2 13% 34% 39% 8%	582
	21st to end	Night	6% 2 10% 40% 34% 9%	657
August	1st to 10th	Night	5% 1 12% 38% 38% 6%	597
	11th to 20th	Night	7% 1 12% 39% 35% 6%	594
	21st to end	Night	7% 1 10% 36% 40% 7%	660
September	1st to 10th	Night	10% 1 10% 35% 40% 4	600
	11th to 20th	Night	12% 3 10% 34% 34% 7%	600
	21st to end	Night	11% 6% 17% 34% 30% 3	720
October	1st to 10th	Night	8% 5% 20% 35% 30% 3	800
	11th to 20th	Night	14% 11% 27% 31% 17% 1	800
	21st to end	Night	21% 13% 28% 26% 11% 1	880
November	1st to 10th	Night	13% 13% 31% 30% 12% 1	820
	11th to 20th	Night	16% 12% 32% 31% 8%	840
	21st to end	Night	21% 15% 32% 24% 8%	907
December	1st to 10th	Night	20% 16% 34% 24% 5%	1050
	11th to 20th	Night	22% 17% 29% 27% 4	1050
	21st to end	Night	24% 19% 31% 23% 4	1155

Page generated at 2026-05-14 00:59:00. Version ac07

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